1. Field of the Invention
The invention relates to a process for preparing 3-glycidyloxypropyltrialkoxysilanes from allyl glycidyl ethers and a trialkoxysilane via platinum-catalyzed hydrosilylation.
2. Description of the Background
3-Glycidyloxypropyltrialkoxysilanes represented by formula (I), shown below, are important industrial intermediates or end products in organosilane chemistry. They are used, inter alia, as adhesion promoters in connection with composite materials; for example, in the paints and glass fibers industry, in foundry engineering and in the adhesives industry. An important role is also played by the 3-glycidyloxypropyltrialkoxysilanes (I) in connection with the coating of optical glasses.
3-Glycidyloxypropyltrialkoxysilanes (I) are prepared by reacting a hydrogen-bearing trialkoxysilane (III) ("H-silane") with allyl glycidyl ether represented by formula (II), shown below, in a hydrosilylation reaction according to the equation EQU CH.sub.2 (O)CHCH.sub.2 OCH.sub.2 CH.dbd.CH.sub.2 +HSi(OR).sub.3 .fwdarw.CH.sub.2 (O)CHCH.sub.2 O(CH.sub.2).sub.3 Si(OR).sub.3.
Here, R is an alkyl radical. When R is methyl, 3-glycidyloxypropyltrimethoxysilane ("GLYMO") is produced. When R is ethyl, 3-glycidyloxypropyltriethoxysilane ("GLYEO") is produced. Byproducts of this reaction include two isomers of (I): EQU CH.sub.2 (O)CHCH.sub.2 --O--CH.sub.2 CH[Si(OR).sub.3 ]CH.sub.3(IV),
which is also referred to as iso-3-glycidyloxypropyltrialkoxysilane, and an eight-membered heterocyclic compound of the formula: ##STR1##
Additional byproducts are glycidyloxytrialkoxysilane, propyltrialkoxysilane, 1-methylvinyl glycidyl ether and tetraalkoxysilane, and also high-boiling components.
Hydrosilylation reactions of H-silanes with compounds containing a C.dbd.C double bond are conducted continuously or batchwise in the presence of a noble metal catalyst. 3-Glycidyloxypropyltrialkoxysilanes (I) are generally prepared in a homogeneous system using hexachloroplatinic (IV) acid as catalyst (see e.g. EP 0 277 023, EP 0 288 286, JP 128763 and DE 21 59 991). Disadvantages of the process with homogeneous catalysis are the difficulty of controlling the temperature during the reaction, the unavoidable task of separating off the catalyst, and the increased formation of byproducts as a result of secondary reactions of the target product under the influence of the homogeneously dissolved catalyst and/or as a consequence of unintended "outliers" in the temperature regime. Especially when catalyst is entrained into the distillative workup stage, the distillation is accompanied by the formation of the abovementioned isomers (IV) and (V) and of dimers and trimers of the 3-glycidyloxypropyltrialkoxysilanes (I). In addition, processes using hexachloroplatinic acid are not chlorine-free, which impairs the quality of the product and promotes, for example, the dissociation of trialkoxysilanes (II) into tetraalkoxysilanes.
EP 0 548 974 describes heterogeneous noble metal catalysts, namely rhodium and platinum, as metals or in the form of compounds which comprise metals as support material and can be used, inter alia, for the preparation of 3-glycidyloxypropyltrialkoxysilanes (I). Also described are noble metal complexes immobilized on nonmetallic supports, such systems likewise being employed for preparing 3-glycidyloxypropyltrialkoxysilanes (I) by hydrosilylation.
EP 0 262 642 describes rhodium-based catalysts for the preparation of 3-glycidyloxypropyltrialkoxysilanes (I), and indeed describes catalysts for both homogeneous and heterogeneous catalysis. The rhodium catalysts for homogeneous catalysis give yields of 3-glycidyloxypropyltrialkoxysilanes (I) of around 70%, while those for heterogeneous catalysis, comprising 5% rhodium on activated carbon as support, give yields of from 70 to 80%. The advantage of the rhodium catalysts over platinum catalysts is said to lie in the reduced sensitivity of the former toward nitrogen compounds, thereby raising the yield of 3-glycidyloxypropyltrialkoxysilanes (I) and reducing the loss of activity suffered by the catalysts.
U.S. Pat. No. 4,736,049 describes, quite generally, hydrosilylation reactions catalyzed homogeneously or heterogeneously with compounds of platinum or with metallic platinum and taking place in the presence of a carboxamide of the general formula RCONR.sub.1 R.sub.2. In relation to supported catalysts mention is made only of platinum on charcoal. The presence of the carboxamide is said, in general terms, to favor the formation of the desired .beta. adduct. According to EP 0 262 642, however, nitrogen compounds, for the specific case of the preparation of 3-glycidylpropyltrialkoxysilanes (I) using platinum catalysts, are said to lessen, quality and are therefore regarded as unfavorable.